In gst-msdk, a mfx session may be shared between different gst
elements, each element tries to set the frame allocator. However, per
the MSDK documation[1], the behavior is undefined if reset the frame
allocator while the previous allocator is in use. Fortunately all
elements use the same frame allocator, so we can avoid to call
MFXVideoCORE_SetFrameAllocator again.
[1]: https://software.intel.com/en-us/node/628430#MFXVideoCORE3
Added a utility method to replace the MemID (interanl VASurfaceID)
associated with the mfxFrameSurface. This is usefull for dmabuf-import
where we need to replace the memID dynamically
https://bugzilla.gnome.org/show_bug.cgi?id=794817
Exporting DRM_PRIME fd to VASurface requires direct
invocation of VA api VACreateSurface with
VASurfaceAttribExternalBufferDescriptor and other
necessary surface attributes.
https://bugzilla.gnome.org/show_bug.cgi?id=794817
Implements msdk frame allocator which is required from the driver.
Also makes these functions global so that GstMsdkAllocator could use
the allocated video memory later and couple with GstMsdkMemory.
GstMsdkContext keeps allocation information such as mfxFrameAllocRequest
and mfxFrameAllocResponse after allocation.
https://bugzilla.gnome.org/show_bug.cgi?id=790752
